When completing wells in earth formations, various fluids are employed in the well for a variety of reasons. Common uses for wellbore fluids, also known as drilling fluids, drilling muds, muds, or drill-in fluids, include: lubrication and cooling of drill bit cutting surfaces during general drilling operations or drilling in a targeted petroliferous formation, suspending dislodged formation pieces from beneath a drill bit and transporting them up the annulus to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability and minimizing fluid loss into the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
Fluids or muds used in drilling a wellbore often include a base fluid, which is commonly water, diesel or mineral oil, or a synthetic compound. Weighting agents (e.g., barium sulfate, barite, etc.) may be added to increase density, and clays such as bentonite, for example, may be added to help remove cuttings from the well and to form a filter cake on the walls of the hole. Other additives may be added to the wellbore fluids that serve specific functions.
During drilling operations, one way of protecting the formation is by forming a filter cake on the surface of the subterranean formation which prevents both the filtration of formation fluids into the wellbore and the loss of wellbore fluids into the formation. Filter cakes are formed when particles suspended in a wellbore fluid coat and plug the pores in the subterranean formation, such that the filter cake acts as a barrier and decreases the fluid permeability of the wellbore. A number of ways of forming filter cakes are known in the art, including the use of bridging particles, cuttings created by the drilling process, polymeric additives, and precipitates. Filter cakes may be formed, for example, during the drilling process by filtration of some amount of the drilling fluid into the formation, or during completion operations when a fluid loss pill (i.e. a viscous pill) used as a spot treatment in a region of the well experiencing fluid loss is injected into a well.
After any completion operations have been accomplished, the filter cake (formed during drilling and/or completion) remaining on the sidewalls of the wellbore may be removed. Although filter cake formation and use of fluid loss pills occur during drilling and completion operations, the barriers can be an impediment to the production of hydrocarbon or other fluids from the well if, for example, the rock formation is still plugged by the barrier. In addition, dislodged filter cake fragments can pose difficulties during production, such as plugging sand screens present within the wellbore. Because filter cake is compact, it often adheres strongly to the formation and may not be readily or completely flushed out of the formation by fluid action alone.
To remove filter cakes and fluid loss pills prior to production, breaker fluids are introduced to the wellbore to remove residual filter cake and fluid loss pills. Breaker fluids may contain, for example, solvents, acids, oxidizers, or enzymes, and may destroy the integrity of a residual filter cake created during the drilling process by removing some or all fluid components, or by degrading the solids that form the filter cake or fluid loss pill. The composition of the breaker fluids depends on the type of drilling fluid and properties of the filter cakes, but breaker fluids are often aqueous solutions. However, when using an oil-based drilling fluid, incompatibilities with aqueous breaker fluids can decrease the efficiency and speed of filter cake removal.
Thus, it is desirable to continually develop compositions and methods to aid and improve the ability to remove filter cakes, with more complete removal, and while minimizing or reducing damage to the formation.